Growth of uniform composition semiconductor crystals



Sept. 15, 1959 N 2,904,512

GROWTH ,OF UNIFORM COMPOSITION SEMICONDUCTOR CRYSTALS Filed July 2, 1956Radio Frequency Oscillator /0 il :2 Forayce Hubbard Horn,

2 30 wmwq His Affomey.

United States Patent GROWTH OF UNIFORM COMPOSITION SEMICONDUCTORCRYSTALS Fordyce Hubbard Horn, Schenectady, N.Y., assiguor to GeneralElectric Company, a corporation of New York Application July 2, 1956,Serial No. 595,503 7 Claims. (Cl. 252-623) The present invention relatesto methods for growing semiconductor single crystals. More particularly,the invention relates to methods for growing semiconductor single,crystals having therein a substantially constant concentration ofimpurities.

Semiconductor materials such as germanium and silicon are, of, greatimportance, particularly in the construction of asymmetricallyconductive devices such as rectifiers and. transistors. Such devicesrequire monocrystalline semiconductor bodies with predictable andsubstantially constant impurity distributions. Generally, thecrystalline bodies. from which such semiconductor devices are formed,are-prepared by subjecting a semiconductor monocrystalline body having aconstant impurity concentration to one or more physical operations such.as alloying or melting and refreezing. Accordingly, there is a greatneed in the electronics industry for methods of growing semiconductorsingle crystals having uniform impurity distributions.

Presently available methods for preparing transistor grade semiconductormaterials are not completely suitable for preparing unconstrained singlecrystals of semiconductor materials having a constant concentration ofimpurities therein immediately suitable for transistor uses. Thus. forexample the Czochralski seed crystal withdrawal method, which is used toproduce high purity semiconductor single crystals, is incapable ofproducing single crystal having constant impurity concentration. This isso because the method involves growing a crystal from a completelymolten bath of semiconductor material impregnated with a givenconcentrationof significant impurities. Since the. segregationcoefiicients of the electrically significant impurities in silicon andgermanium, are generally markedly less than unity, the concentration ofsuch impurities in the melt from which a crystal is grown progressivelyincreases. as a crystal is grown, therefrom, resulting in aprogressively increasing concentration of these impurities in the growncrystal.

The zone melting method of forming semiconductor bodies having thereinuniformly distributed impurities is not well adapted to the productionof single crystals. It is extremely difiicnlt to form a single crystalof a semiconductor material in a crucible as is done in zone-melting.Even. such difficulties arev overcome and a single crystal is grown, thegrowth of the crystal Within the confines of a crucible or boat resultsin the introduction of strains in the crystal due to its growth underconstraint. fuch strains are intolerable, and must be removed byannealing, which requires further heat treatment of the crystal for manyhours or even days.

Accordingly, one object of the invention is to provide a method for theunconstrained growth, of single crystals of semiconductor materialshaving therein a constant concentration of electrically significantimpurities.

' Another object ofthe invention is toprovide a method for growingsingle crystals of semiconductor materials having therein a constantconcentration of electrically significant impurities and which requireno further treatment to render them suitable for use in transistordevices.

In accord with my invention, I prepare unconstrained single crystals ofsemiconductor materials having therein a uniform distribution ofelectrically significant im purities having a segregation coefficientless than unity by growing a single crystal by seed crystal withdrawalfrom a substantially constant volume molten zone of a semiconductormaterial maintained, within a crucible. As the single crystal is grown,the molten zone isconstantly replenished by continuously meltingsolidified semiconductor material within the crucible. Crystals grown bythis method have a uniform distribution of impurities therein, areunconstrained, and are ready for-immediate use in the preparation ofsemiconductor devices.

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself together with furtherobjects and advantages thereof may best be understood. by reference tothe attached drawing in which the sole figure is a cross-sectional viewof an apparatus suitable for use in the practice of the invention.

The illustrated apparatus. includes a refractory furnace envelope 1having a closed base 2 and an open flanged upper end 3. Envelope 1 ispreferably cylindrical in shape and is much longer than its diameter. Inthe upper portion of the cylinder of furnace envelope 1 thereis locatedan input feeder tube 4 disposed at an acute angle to the axis ofenvelope 1 and closed by a spring biased closure member 5 having acentral aperture 6 therein, closed by a ball check valve 7. Withincylindrical envelope 1 is located a cylindrical crucible 8. having adepth dimension at least 1 /2 times its diameter which is guided to acentral position by a sleeve member 9 and. a centering spring 19.Crucible 3 is supported upon. a suitable annular, outwardly flangedsupport member 11 conveniently packed with a mass of quartz wool 12.Furnace envelopev 1, spacer 9, crucible 8 and support member 11 are conveniently fabricated from quartz.

The upper, open end of envelope 1 is fastened to a closure plate 13which is spaced from a pair of supported plates 14. and 15 by means ofvertical rods 16. and 16.. Plates 13, 14. and 15 comprise a part of theapparatus for regulating the movement of movable parts of the apparatusas will be more fully described hereinafter. Closure plate 13' has anaperture 17 therein for the passage of a crystal drawing mandrel 18'-therethrough. An airtight seal 19 maintains a closed atmosphere withinfurnace envelope 1 and allows for the passage of mandrel 18therethrough. Mandrel 18 is fastened by coupling 20 to a threaded shaft21 which passes through supporting plate 14 and is connected through asuitable stepdown gear mechanism 22 to gear ratio control box- 23.Control box 23 serves as a distributor for driving energy suppliedby'electric motor 24 and contains suitable. clutches for thedrive-shafts operated therefrom so that they may be turned manually whendesired. A second drive shaft 25 drives coil holder 26 from gear controlbox 23 and raises and lowers radio frequency inductance heating coils 27and 2.8 with respect to crucible 8 within furnace envelope 1. Gearcontrol box 23 adjusts the relative speed ratio of mandrel 1.8 and driveshaft 25 so that mandrel 18 is raised at a first preselected rate andinduction heating coils 27 and 28 are lowered at a second preselectedrate.

A mass 29 of semiconductor material, suchv as germanium or silicon islocated in crucible 8. A zone 30 of semiconductor mass 29 is maintainedin the moltenstate and a single crystal 31 of semiconductor material, isgrown therefrom by seed crystal withdrawal. Ingot 31 is. grown upon seedcrystal 32 which is maintained in chuck 33 attached to mandrel 18. Radiofrequency energy for heating semiconductor 29 in crucible 8 is suppliedto RF induction coils 27 and 28, which are electrically in series, bymeans of conductors 34 and 35 which are connected to a radio frequencyoscillator. During the entire operation a protective gas, as for exampleargon, is caused to flow into envelope 1 through inlet conduit 37 andescapes through aperture 38 in the base of envelope 1.

In practicing the invention a mass 29 of semiconductor material having aknown impurity content is placed within crucible 8. Convenientlysemiconductor mass 29 may be prepared by zone melting and may have thecharacteristics of intrinsic semiconductor, with a resistivity of inexcess of 30 ohm centimeters in the case of germanium and 100 ohmcentimeters in the case of silicon. Radio frequency power is suppliedthrough conductors 34 and 35 to induction heating coils 27 and 28 tocompletely melt the charge of semiconductor material within crucible 8.Conveniently mass 29 may be preheated by a resistance coil (not shown)to make the semiconductor more conductive and more receptive toinduction heating. If the material being melted is germanium, thetemperature must be raised above 941 C., and if the semiconductormaterial utilized is silicon, the temperature must be raised above 1420C.

When the entire mass 29 of semiconductor material in crucible 8 has beenmelted, the power supplied to induction coils 27 and 28 is abruptlylowered so that the entire crucible full of semiconductor materialsolidifies nondirectionally but is maintained above a temperature atwhich the semiconductor remains plastic. It is an important feature inthe practice of the invention that once the semiconductor mass 29 incrucible 8 has been melted, and allowed to refreeze, it is maintainedabove a temperature at which the semiconductor remains plastic. This isnecessary to prevent expansion upon complete cooling, and a consequentfracture of the crucible. Thus, for example, if the material utilized isgermanium, the refrozen material within the crucible is maintained at atemperature in excess of 600. If the material is silicon, it ismaintained at a temperature in excess of 1000 C. As soon assemiconductor material 29 has refrozen, coil holder 26 is manuallyadjusted so that heater coil 27 is adjacent the upper surface of thematerial within the crucible. The power to induction heating coils 27and 28 is then increased until a molten zone 30 forms at the top ofsemiconductor mass 29. A molten zone forms only at the top of thecrucible because the number of turns on induction coil 27 is muchgreater than the number of turns for coil 28, and the heat energysupplied to the upper end of the frozen material within crucible 8 issufficient to melt the semiconductor contained therein, while theremainder of the frozen ingot remains solid. The thickness of moltenzone 30 may be adjusted by changing the number of turns upon radiofrequency coil 29, or by increasing or decreasing the power thereto. Thethickness of zone 31 is conveniently adjusted to from approximately /2to of the diameter of the crucible used.

If semiconductor material 29 is high-purity or intrinsic semiconductor,molten layer 30 is then inoculated with the concentration of asignificant activator impurity sufficient to supply to crystalline ingot32 grown in equilib rium therewith the desired concentration of theimpurity. This amount for small values of K may be calculated from thesimplified relationship C=KC (1) where Mandrel 18 is next manuallylowered until seed crystal 32 is in contact with the surface of moltenzone 30. The seed crystal is left in this position until it is observedto fuse with the molten zone. Mandrel 18 and drive shaft 25 are thenconnected directly to gear ratio box 23 as for example by clutchestherein, and driving power is then supplied to gear ratio control 23 tocause mandrel 18 to slowly rotate and rise. The speed of rotation ofmandrel 18 and hence seed crystal 32 is not critical, since rotation isnot necessary. Conveniently, the mandrel may be rotated at a speed ofrevolutions per minute. The threads upon threaded shaft 21 mesh with aninternally threaded coupling 36 on plate 14 and cause mandrel 18 andseed crystal 32 to slowly rise. The rate of pulling or the rate of riseof seed crystal 31 may be any rate less than 6 inches per hour and isconveniently chosen in accord with the time cycle desired. As mandrel18, and consequently seed crystal 32 rises, a monocrystalline ingot 31of semiconductor material impregnated with a concentration C of thechosen significant impurity element is grown from molten zone 30.

As crystal 31 is grown the amount of semiconductor material withinmolten zone 30 would be decreased if no compensation were made for thematerial lost to crystal 32. Therefore, by means of gear ratio box 23,drive shaft 25 which is clutched to operate automatically at the sametime mandrel 18 is so clutched is geared to rotate and to lowerinduction heating coils 27 and 28 at a speed which is related to thespeed at which seed crystal 32 is withdrawn by a ratio approximatelyinversely proportional to the ratio of the cross-sectional area ofcrucible 8 and the cross-sectional area of monocrystalline ingot 31.

More particularly the ratio of these rates is governed by therelationship where P=the rate of crystal withdrawal L=the rate at whichthe molten zone is lowered D=the diameter of the crucible d=the diameterof the growing crystal It will be appreciated that in order to growunconstrained single crystals in accord with the invention d must alwaysbe less than D.

By virtue of this regulation, molten zone 30 is continually maintainedat a substantially constant volume. Since the vast majority of thesignificant activator impurities added to molten zone 39 are segregatedout of growing crystal 31, the concentration of significant activatorimpurity material within molten zone 30 remains substantially constant.This is particularly true for segregation coefficients of less than0.05. As is well known to the art, the segregation coefiicient of animpurity in a semiconductor grown from a liquid melt with which it is inequilibrium is defined as the ratio of the concentration of the impurityin the growing crystal to the concentration of the impurity within themelt. The method of the invention is suited for growing semiconductorcrystals having therein a constant concentration of significantactivator impurities having segregation coeflicients less than unity. Itis, however, preferably practiced with impurities having segregationcoefficients less than 0.05 but greater than 0. Since the volume ofmolten zone 30 remains substantially constant, and since theconcentration of significant activator impurities therein remainsubstantially constant, the concentration of activator impurities withingrowing crystal 31 is substantially constant until the ingot is grown tofull size, and only a small amount of residual semiconductor remains inthe bottom of crucible 8.

In this respect it should be noted that the diameter of bottom flange 2on furnace envelope 1 is smaller than the inside diameter of RF coils 27and 28 thus permitting the coils to be lowered below the lowest.extremity of furnace 1. to grow an ingot from substantially. the entirecharge of semiconductor within the crucible.

The process of the invention may be practiced, with additions ofnumerous impurities to semiconductors such as germanium or silicon. Asused herein, the term impurity does not connote. an undesirableinclusion within the semiconductor body but connotes an electricallysignificant inclusion which, because of its presencein small amounts, isresponsible for the highly desirable semiconducting properties ofsemiconductive materials. Such impurities are divided into two principalgroups which are, electrically significant. These groups are theacceptors of, group III of the periodic table which include aluminum,gallium and indium, and the donors. of group V of the periodic tablewhich include phosphorus, arsenic and antimony. All of these impuritieshave. segregation coefiicients in both germanium and silicon less than.unity. A. third but less-known group of, electrically significantimpurities in germanium and silicon are the elements which introducedeep trapping levels into the semiconductor energy scheme. Thesematerials include iron, nickel, cobalt, manganese, zinc and gold. Allofthese materials have segregation coefiicients in, germanium, and siliconless. than .001 and are, therefore, ideally suited for-use in thepractice of this invention. The characteristics of these materials ingermanium may be found in. an. article entitled Scattering of CarriersFrom Doubly Charged Impurity Sites in Germanium, by W. W. Tyler and H.H. Woodbury, vol. 102, Physical Review, page 647, May 1, 1956, and thereferences cited therein.

In order to grow semiconductor single crystals having therein asubstantially constant concentration of electrically significantimpurities in accord with the method of the. invention, it is preferablethat; the segregation c,o.--

eflicient of the selected impurity in the semiconductor material be.lower than 0.05. Thus, for example, when indium, having a segregationcoefiicient of 0.001,v arsenic having a segregation coeflicient of 0.04or antimony having a segregation coefficient of 0.003. are addedto amolten zone of a germanium melt and the method practiced. in accord withthe foregoing disclosure, a single crystalline semiconductor ingot maybe produced having less than a variation in the concentration ofimpurities from one end to the other. Such a condition is ideal.However, semiconductor ingots of germanium having an impuritydistribution well within the specifications and needs for the productionof semiconductor devices such as transistors and rectifiers may be grownin accord with this invention utilizing, as well as the, aforementionedimpurities, aluminum, gallium and phosphorus all of which have asegregation coeflicient in germanium equal tov 0.1. While a crystalgrown from a germanium melt in accord with the invention having a moltenzone inoculated with aluminum, gallium or phosphorus does not have acompletely uniform distribution of these impurities from one end to theother, the variation is not great enough to detract from the usefulnessof the germanium as a source of material for the production ofsemiconductor devices.

In silicon, indium having a segregation coefiicient of 0.0003, galliumhaving a segregation coefiicient of 0.004, aluminum having a segregationcoeificient of 0.002, and antimony having a segregation coeflicient of0.02, may all be used in the practice of the invention for theproduction of silicon crystals having a substantially onstantconcentration of these impurities along a single crystal ingot. Witharsenic or phosphorus in silicon the situation is somewhat difierent.Both of these materials have a segregation coefiicient in silicon equaltov 0.3. Because of this rather high segregation coefficient, if thevolume of molten zone 30 is maintained constant as crystal 31 is growntherefrom, the concentration of these impurities of; the grown crystalfalls substantially, as the ingot. is grown. Such a condition, however,may be compensated 6 for, in the case of these two impurities, by a verysimple. modification. This may be accomplished by the simple expedientof progressively decreasing the thickness of molten zone 30 as ingot 31,is grown therefrom. The thickness of molten zone 30 is decreased inaccord with the relationship l=l .kx (3) where.

l =the thickness of the molten zone at; anytime l =the initial thicknessof the molten zone lc=the segregation coefiicient of the. impurityutilized x=lthe length of the. semiconductor material 29. in cruel:

ble 8.

The thickness 1 of molten zone 30 may be caused to be progressivelydecreased as a crystal is grown therefrom by any one of several simpleexpedients. Thus, for example, the thickness may be decreased by causinginduction coil 27 to progressively lag farther and farther behind in itsdownward progress along with the decreasing height of semiconductormaterial 29 in crucible 8. This may be accomplished by progressivelydecreasing the pitch of the threads along drive shaft 25 so that coilcontrol mechanism 26 is lowered at a progressively decreasing rate.Alternatively, the radio frequency electrical energy supplied to coil 27through conductor 35 and 36 may be progressively decreased; thussupplying a lesser amount of heat energy to cause the formation ofmolten zone 30.

The amount of significant activator impurities which may be added tomolten zone 30 in the practice ofthe invention are not critical. As apractical matter for single crystal growth there is no reason for addingto the molten layer an amount of impurity which will result in aconcentration therein greater than the quantity .e impurity in, thesemiconductor.

This. amount results in the presence, within the growing semiconductorcrystal, of the maximum amount of mate.- rial soluble therein. Theaddition of an amount of activator impurity to the molten zone. whichresults. in a, higher concentration therein does not result in theincorporation of a greater amount or concentration of activator impurityWithin the growing semiconductor crystal. In fact, as greater amounts ofactivator impurities are added to the molten zone, it eventually becomes difficult to grow single crystals of semiconductor materialstherefrom.

There is, however, no minimum amount which acts. as a limit to theamount of activator impurity which may be added to molten zone 30 in thepractice of the. invention. The addition of the smallest measurabletrace of activator impurity to molten zone 30 results in a small butnevertheless detectable amount of activator impurity being deposited inthe growing crystal. This may be readily appreciated when it isrealized, that, in semiconductors such as germanium and silicon, theaddition. of traces of electrically significant activator impurities assmall as one part per million has a decided eifect upon the electricalcharacteristics of the semiconductor crystal.

It is, in fact, even possible to practice the invention without addingany significant activator impurity to, molten zone 30 through closure 6prior to growing a crystal therefrom. This may be done by thealternative of using in the case of impurities having a segregation co.-efiicient from 0.5 to 1.0 as a starting material, germanium of siliconhaving a preselected, concentration of electrically significantactivator impurities. therein. The op; eration, in this case, results ina uniform distribution in all but the first and last grown parts of thesingle crystalline semiconductor ingot of a concentration of activatorimpurities equivalent to the concentration of the activator impuritiesin the starting material added to crucible 8. In this case, the firstportion of the ingot grown has a concentration of activator impuritiesless than the concentration of activator impurities within the chargematerial 29. However, as the crystal is grown, the concentration ofactivator impurities within molten zone 30 rapidly increases, due to therejection of the impurity by the growing crystal at a rate inverselyproportional to the segregation coefficient of the impurity in thesemiconductor. When, however, the concentration of activator impuritywithin molten zone 30 rises to the value of C /K where C is theconcentration of the impurity in the starting material and K is thesegregation coefficient of the activator material in the semiconductor,the concentration of the activator impurity in the remainder of thegrown ingot is substantially constant and is equivalent to theconcentration of the activator impurity in the starting material. As apractical matter, the first grown portion of the ingot which exhibits alesser concentration than that of the starting material is a very smallportion of the ingot amounting at most to two zone widths, and mayreadily be detected and discarded.

In the case of impurities in germanium and silicon having a segregationcoefficient less than 0.5 and preferably less than 0.01, singlecrystalline ingots having a substantially constant concentration ofimpurities therein may be grown without inoculation of the molten zoneby an added step of directional cooling of the initial molten mass ofsemiconductor 29 before the molten zone is formed. Thus, after theinitial charge of germanium in crucible 8 has been melted and inductionheating coil 27 is adjacent the upper portion of the moltensemiconductor within the crucible, the power to induction heating coils27 and 28 is reduced gradually rather than abruptly so that thesemiconductor material 29 within crucible 8 freezes directionally fromthe bottom of the crucible to the top thereof at a rate of from toinches per hour. This slow directional freezing causes the impuritiespresent within the molten semiconductor to be rejected from thedirectionally freezing material within the crucible, and causes aconcentration of the rejected impurity in the last-frozen portion of thesemiconductor within the crucible. Since this last-frozen portion is atthe top of the crucible, when the power to induction heating coils 27and 28 is again increased to form a molten zone at the top of thecrucible this molten zone already contains a high concentration ofactivator impurities and a single crystalline ingot having asubstantially constant concentration of impurities therein may be growntherefrom without a further inoculation of the molten zone.

Accordingly, it is evident that the invention may be practiced in twoalternative methods. In one method, substantially intrinsic, highlypurified semiconductor material is used as a starting material and apreselected activator impurity is added to the molten zone. Theconcentration of activator impurity which is deposited in the growingzone of crystalline ingot is equal to C/K. In the other alternativemethod, the starting material utilized already contains a concentrationof one or more activator materials and no addition is made to the moltenzone. In this case, the greater portion of the single crystalline ingotgrown in accord with the method has a concentration of activatorimpurities substantially equal to that of the concentration of activatormaterials in the starting material.

While the broad concept of the practice of the invention has been setforth hereinbefore, the following specific examples are given toillustrate the exact procedure which may be followed in the growing ofcertain semiconductor crystals having specific constant concentrations"of designated impurities therein. These examples are a: given forillustrative purposes only and are not to be construed in a limitingsense.

Example 1.Tl1e structure illustrated in the drawing is used. .In thisstructure, furnace envelope 1 has a diameter of 4 inches and is 18inches long. Crucible 8 has an inside diameter of approximately 1%inches and is approximately 3 inches long. The system is first openedand a seed crystal of single crystalline germanium is inserted in thechuck. Crucible 8 is filled with 300 grams of zonemelted germaniumhaving a resistivity in excess of 40 ohm centimeters. The system is thenclosed and flushed with argon gas at a pressure of 1 atmosphere forapproximately 5 minutes. Power is then supplied to induction heatingcoils 27 and 28 with induction heating coil 27 located adjacent thebottom of crucible 8. A molten zone is formed at the bottom of crucible8 and the heater is raised manually until the entire charge within thecrucible is molten or at a temperature above 941 C. With inductionheating coil 27 adjacent the upper surface of the material in thecrucible, the power supplied to the coils is slowly reduced until thematerial in the crucible solidifies and falls to a temperature ofapproximately 650 C. Power to the induction heating coil is againincreased, causing the formation of a molten zone at the top of thesolidified germanium in the crucible. Power is adjusted to cause themolten zone to have a thickness of approximately inch, and maintained atthat level. Ball check valve 7 is then removed from closure 6 and 330milligrams of antimony are inserted into molten zone 30. Mandrel 18 isthen manually lowered until contact is made between the molten zone ofgermanium and the seed crystal. The seed crystal is maintained in thisposition until the crystal is observed to begin to melt. Mandrel 18 anddrive shaft 25 are then placed on automatic feed by operating suitableclutches in gear box 23, and the seed crystal is rotated at a speed ofapproximately rpm. and withdrawn at a rate of 4 inches per hour. Coils27 and 28 and molten zone 30 are lowered as the seed crystal iswithdrawn at a rate of 2% inches per hour. At the end of 50 minutes agrown crystal 3 /2 inches long is withdrawn from the melt leaving theresidue of approximately 70 grams of unused germanium at the bottom ofthe crucible. This crystal is then removed from the chuck and is foundto have a resistivity of approximately 0.01 ohm centimeter throughoutits length.

Example 2.-A single crystal of germanium having therein a substantiallyconstant concentration of arsenic and having a resistivity ofapproximately 0.01 ohm centimeter is formed using the same procedure asutilized in Example 1 with the addition of 16 milligrams of arsenic tothe molten zone instead of the antimony added in Example 1.

Example 3.A single crystal of germanium having therein a substantiallyconstant concentration of phosphorus therein along its length and havinga resistivity of approximately 0.2 ohm centimeter is formed using theprocedure described in Example 1 but adding to the molten zone 1.0milligram of phosphorus instead of the antimony addition of Example 1.

Example 4.A single crystal of germanium having therein a substantiallyconstant concentration of indium and having a resistivity ofapproximately 1 ohm centimeter is formed using the procedure set forthin Example 1 but adding to the molten zone 2.5 milligrams of indiuminstead of the antimony of Example 1.

Example 5 .A single crystal ingot of germanium having therein a constantconcentration of aluminum and having a resistivity of approximately 0.1ohm centimeter is formed using the procedure of Example 1 but adding tothe molten layer 5 milligrams of aluminum instead of the antimony addedin Example 1.

Example 6.A single crystal of germanium having therein a constantconcentration of gallium and a resistivity of approximately 4 ohmcentimeters is formed using the procedure of Example 1. but adding, to.the molten zone 7.0 milligramsof gallium.

Example 7;- Tlie structure illustrated. in Figure 1 of the drawing isused. The. dimensions of. the structure are the same asthose set: forthin Example: 1. The system is first opened and a seed crystal of. singlecrystal silicon is inserted in the chuck. Crucible 8 is then filled with150 grams ofzone melted silicon having a resistivity in excess of 100ohm centimeters. The system is then closed and flushed with argon gas ata pressure of 1 atmosphere for approximately minutes. After thisperiod'the argoniscontinually fed into the system to maintain aprotective atmosphere. Power isnext supplied to inductionheating coils 27 and.28 with induction heating coil 27 located adjacent thebottom ofcrucible 8.- A molten zone is formed at the bottom of crucible 8, andthe heateris raised manually. until the entire charge withinthe-crucible is molten, and at a temperature in excess of 1420 C. Withinduction-heating coil 27 adjacent the upper surface of the material inthe crucible, the power supplied tothe coilsis slowly reduced until thematerial in the crucible solidifiesand falls to'a temperature ofapproximately 1000 C. Power to the induction heating coil is againincreased until a molten zone forms at the top of thesolidified materialwithin crucible 8. The power is increased until the molten zone has athickness of approximately inch; Ball check valve 7 is then removed fromclosure 6 and 1.2 milligrams of antimony are inserted intomoltenzone-30. Mandrel 18 is-then manually lowered'until. contact is madebetween the seed crystal ofsilicon and the molten silicon. The seedcrystal is maintained inthis. position untilthe crystal is ob.- servedto begin to melt. Mandrel 18- and drive shaft25 are then connected toautomatic feed through gear box 23 and the seedcrystal is rotated andwithdrawn at a rotation rate of approximately 100 revolutions per minuteand a rate of withdrawal of approximately 4 inches per hour. Molten zone30- progressively falls and induction heating coil 27' is automaticallylowered at a rate of 2% inches per hour by drive-shaft 25. At the end of50 minutes the grown crystal approximately 3 /2 inches in length andinch in diameter is withdrawn from the melt, leaving the residueofapproximately 35 grams of unused silicon at the bottom of thecrucible. This crystal is then removed from the chuck and is found tohave a resistivity of approximately 2 ohm centimeters.

Example 8.A single crystal of silicon having therein a substantiallyconstant concentration of aluminum and having a resistivity ofapproximately 1 ohm centimeter is formed using the same procedure as isutilized in Example 7 with the addition of 1.4; milligrams of aluminumto the molten zone of silicon instead of the antimony addition ofExample 7;

Example 9.-A single: crystal of silicon having therein a substantiallyconstant concentration of gallium along its length and having aresistivity of approximately 0.1 ohm centimeter is formed using theprocedure described in Example 7' but adding to the molten zone ofsilicon 8 milligrams of gallium instead of the antimony addition ofExample 7.

Example 10.--A single crystal of silicon having therein a substantiallyconstant concentration of indium and having a resistivity ofapproximately 4 ohm centimeters is formed using the procedure set forthin Example 7 but adding to the molten zone of silicon approximately 8milligrams of indium instead of the antimony added in Example 7.

Semiconductor single crystals of silicon or germanium impregnated withthe hereinbefore described acceptor elements or donor elements producedin accord with the invention have utility as sources of semiconductivematerials for the production of semiconductor devices such as rectifiersor transistors. Thus, for example, an ingot of germaniumorsiliconproduced in accord with the in vention and having asubstantially uniform concentration of a particular activator impuritytherein may. be cut transversely into thin slabs approximately 0.05 inchthick which thin slabs may. then be cut into square or rectangua larwafers which may housed in the production of rectifi'ers and transistorsof the fused junction type as disclosed. and. claimed incopendingapplication Serial No. 187,478 filed September 29, 1950, now abandoned,and assigned to the same assignee, as the present application.

In additionv to the formation of semiconductor single crystals havingtherein-a substantially uniform distribu= tion of a single activatorimpurity, as is described hereinbefore, it. is. obvious that.semiconductor crystals, may be formed having. therein-a substantiallyuniform concentra: tion of two or more activator impurities. This is dueto the, fact that the segregation coeflicients and the solubilities. ofelectrically significant activator, impurities in germanium and siliconare independent of the presence of. other impurities therein. Thus, inthe practiceof the invention, one may add to the constant volume moltenzone within the crucible, a quantity of two or more actiI- vatorimpurities, and. the process carried on in substantiallyidenticalmanner, with the production of germanium or silicon ingotshaving;therein substantially constant concentration of two or moreactivator impurities. Such ingotsv may then be cut into rectangularcross-sectioned needles or rods of. germanium or silicon and used as astartingmaterial for the. production of rectifiers and, transistors inaccord with the local fusion technique described and claimed incopending application of Robert N. Hall, Serial No. 516,637, filed June20, 1955, now US. Patent 2,822,309, and assigned; to the. same assigneeas the pres.- ent. application.

While the invention has been described with respectto certain practicesthereof, it will be. appreciated thatmany variations. and. changes willimmediately occur to those skilled. in the art. Accordingly, I intend bythe ap..- pended claims to cover all such modifications that fall withinthe true spirit and scope of the foregoing dis.- closure.

What I claim asnew and desire to secure by Letters Patent of the UnitedStates is:

1. The method of growing unconstrained single crystals of asemiconductor materialv selected from the group consisting of germaniumand silicon having a substantially constant concentration ofelectrically significant activator impurities. therein which methodcomprises placing a quantity of the semiconductor material in a verticalcrucible the depth of which is greater than its diameter, raising thetemperature of the semiconductor above. its fusion point to cause it tobecome molten, lowering the temperature of the semiconductor to atempera: ture below its fusion point but above the temperature at whichthe semiconductor remains plastic to cause the semiconductor to freezeinto a solid but plastic mass, raising the temperature of a zone of thefrozen semiconductor mass adjacent to and bounded by the upper surfacethereof abovethefusion temperature of the semiconductor whilemaintaining the remainder of the mass at atemperature below the fusionpoint thereof but above the temperature at which the semiconductorremains plastic to establish a molten zone adjacent the upper surfacethereof, contacting the surface of the molten zone with a seed crystalof the semiconductor, and growing a single crystalline ingot ofsemiconductor by seed crystal withdrawal from the molten zone whilereplenishing the molten zone from the unmelted portion of saidsemiconductor mass to maintain the molten zone at a substantiallyconstant volume.

2. The method of growing unconstrained single crystals of asemiconductor material selected from the group consisting of germaniumand silicon having a substantially constant concentration of anelectrically significant activator impurity therein which methodcomprises placing a quantity of the semiconductor material in a verticalcrucible the depth of which is greater than its diameter,

raising the temperature of the semiconductor above its fusion point tocause it to become molten, lowering the temperature of the semiconductorto a temperature below its fusion point but above the temperature atwhich the semiconductor remains plastic to cause the semiconductor tofreeze into a solid but plastic mass, raising the temperature of a zoneof semiconductor adjacent to and bounded by the upper surface of thefrozen mass while maintain ing the remainder of the mass at atemperature below the fusion point thereof but above the temperature atwhich the semiconductor remains plastic to cause the creation of amolten zone of semiconductor which comprises only a portion of the totalmass of semiconductor within the crucible, contacting the surface of themolten zone with a seed crystal of the semi-conductor, and growing asingle crystal of semiconductor from the molten zone by seed crystalwithdrawal therefrom while progressively melting the remaining solidmass of semiconductor in the crucible to replace the semiconductorremoved from the molten zone by growth of the crystal therefrom andmaintaining the molten zone at a substantially constant volume.

3. The method of growing unconstrained single crystals of asemiconductor material selected from the group consisting of germaniumand silicon having a substantially constant concentration of anelectrically significant activator impurity therein which methodcomprises placing a quantity of the semiconductor material in a verticalcrucible the depth of which is greater than its diameter, raising thetemperature of the semiconductor above its fusion point to cause it tobecome molten, lowering the temperature of the semiconductor to atemperature below its fusion point but above the temperature at whichthe semiconductor remains plastic to cause the semiconductor to freezeinto a solid but plastic mass, raising the temperature of a zone of thefrozen mass of the semiconductor adjacent to and bounded by the uppersurface thereof while maintaining the remainder of the mass at atemperature below the fusion point thereof but above the temperature atwhich the semiconductor remains plastic to cause the creation of amolten zone of semiconductor adjacent to and bounded by the uppersurface thereof which comprises only a portion of the semi-conductor inthe crucible, and to establish a liquid-solid interface between thelower surface of the molten zone and the unmelted mass of semiconductorremaining in the crucible, contacting the upper surface of the moltenzone with a seed crystal of the semiconductor, and growing a singlecrystalline ingot of semiconductor material by seed crystal withdrawalfrom the molten zone while progressively lowering the interface betweenthe molten zone and the remaining unmelted semiconductor material toreplenish the molten zone with a quantity of semiconductor substantiallyequal in volume to the amount lost by the growth of the single crystaltherefrom and maintain the molten zone at a substantially constantvolume.

4. The method of growing unconstrained single crystals of asemiconductor material selected from the group consisting of germaniumand silicon and having a substantially constant concentration ofelectrically significant activator impurities therein which methodcomprises placing a quantity of the semiconductor material in a verticalcrucible the depth of which is greater than its diam eter, thesemiconductor having a concentration of activator material therein equalto the concentration of activator material desired in the grown singlecrystal, raising the temperature of the semiconductor above its fusionpoint to cause it to become molten, rapidly lowering the temperature ofthe semiconductor to a temperature below its fusion point but a abovetemperature at which the semiconductor remains plastic to cause thesemiconductor to freeze non-directionally into a solid but plastic mass,raising the temperature of a zone of the frozen semiconductor ,massadjacent to and bounded by the upper surface thereof above the fusiontemperature thereof while maintaining the remainder of the frozen massat atemperature below its fusion. point but above the temperature atwhich the semiconductor remains plastic to establish a molten zoneadjacent the upper surface thereof, contacting the surface of the moltenzone with a seed crystal of the semiconductor, and growing a singlecrystalline ingot of semiconductor by seed crystal withdrawal from themolten zone while replenishing the molten zone from the unmelted portionof the semiconductor mass to maintain the molten zone at a substantiallyconstant volume.

5. The method of growing unconstrained single crystals of semiconductormaterials selected from the group consisting of germanium and siliconand having a substantially constant concentration of electricallysignificant activator impurities therein which method comprises placinga quantity of the semiconductor material in a vertical crucible thedepth of which is greater than its diameter, raising the temperature ofthe Semiconductor above its fusion point to cause it to become molten,lowering the temperature of the semiconductor to a temperature below itsfusion point but above the temperature at which the semiconductorremains plastic to cause the semiconductor to freeze into a solid butplastic mass, establishing a molten zone of semiconductor adjacent toand bounded by the upper surface of the frozen mass of semiconductorwhile maintaining the remainder of the semiconductor in a solid butplastic state, adding to the molten zone a preselected quantity of anelectrically significant activator impurity for the semiconductor havinga segregation coefficient in the semiconductor less than unity,contacting the surface of the molten zone with a seed crystal of thesemiconductor, and growing a single crystalline ingot of semiconductorby seed crystal withdrawal from the molten zone while maintaining themolten zone at constant volume by replenishment from the solid mass ofsemiconductor within the crucible.

6. The method of growing unconstrained single crystals of asemiconductor material selected from the group consisting of germaniumand silicon having a substantially constant concentration of anelectrically significant activator impurity having a segregationcoefficient in the semiconductor less than unity therein which methodcomprises, placing a quantity of the semiconductor material in avertical crucible, the depth of which is greater than its diameter, thesemiconductor having a concentration of activator materials thereinequal to the concentration of activator material desired in the grownsingle crystal; raising the temperature of the semiconductor above itsfusion point to cause it to become molten; slowly lowering thetemperature of the semiconductor to directionally cool the semiconductorwithin the crucible from the bottom thereof to concentrate the activatorimpurities contained therein in the last frozen portion of thesemiconductor melt adjacent the upper surface thereof; maintaining thefrozen semiconductor mass at a temperature below its fusion point, butabove the temperature at which the semiconductor remains plastic;raising the temperature of a zone of the frozen semiconductor massadjacent to and bonded by the upper surface thereof above the fusiontemperature thereof while maintaining the remainder of the frozen massat a temperature below its fusion point but above the temperature atwhich the semiconductor remains plastic to establish a molten zoneadjacent the upper surface thereof; contacting the surface of the moltenzone with a seed crystal of the semiconductor; and growing a singlecrystalline ingot of semiconductor by seed crystal withdrawal from themolten zone while replenishing the molten zone from the unmelted portionof the semiconductor mass to maintain the molten zone at a substantiallyconstant volume.

7. The method of growing unconstrained single crystals of silicon havinga substantially constant concentration of electrically significantactivator impurities therein selected from the group consisting ofarsenic and phosphorus which method comprises, placing a quantity of thesemiconductor material in a vertical crucible the depth 13 of which isgreater than its diameter; raising the temperature of the semiconductorabove its fusion point to cause it to become molten; lowering thetemperature of the semiconductor to a temperature below its fusion pointbut above the temperature at which the semiconductor remains plastic tocause the semiconductor to freeze into a solid plastic mass;establishing a molten zone of semiconductor adjacent to and bonded bythe upper surface of the frozen mass of the semiconductor whilemaintaining the remainder of the semiconductor mass in a solid butplastic state; adding to the molten zone a preselected quantity of thechosen electrically significant activator impurity; contacting thesurface of the molten zone with a seed crystal of the semiconductor;growing a single crystalline ingot of the semiconductor by seed crystalWithdrawal from the molten zone; continuously replenishing the moltenzone from the solid mass of semiconductor within the crucible at a ratewhich continuously decreases the thickness of the molten zone so thatthe thickness of 14 the molten zone at any time is defined by therelationship 1=l -kx where l=the thickness of the molten zone at anytime i =the initial thickness of the molten zone 5 k=the segregationcoeflicient of the impurity utilized x=the length of the semiconductormaterial remaining in the crucible at the given time References Cited inthe file of this patent UNITED STATES PATENTS OTHER REFERENCES Bell Tel.Lab. Inc. and Western Electric Inc., Transistor Technology, July 1952,Parts I and II.

4. THE METHOD OF GROWING UNCONSTRAINED SINGLE CRYSTALS OF ASEMICONDUCTOR MATERIAL SELECTED FROM THE GROUP CONSISTING OF GERMANIUMAND SILICON AND HAVING A SUBSTANTIALLY CONSTANT CONCENTRATION OFELEFTRICALLY SIGNIFICANT ACTIVATOR IMPURITIES THEREIN WHICH METHODCOMPRISES PLACING A QUANTITY OF THE SEMICONDUCTOR MATERIAL IN A VERTICALCRUCIBLE THE DEPTH OF WHICH IS GREATER THAN ITS DIAMETER, THESEMICONDUCTOR HAVING A CONCENTRATION OF ACTIVATOR MATERIAL THEREIN EQUALTO THE CONCENTRATION OF ACTIVATOR MATERIAL DESIRED IN THE GROWN SINGLECRYSTAL, RAISING THE TEMPERATURE OF THE SEMICONDUCTOR ABOVE THE FUSIONPOINT TO CAUSE IT TO BECOME MOLTEN, RAPIDLY LOWERING THE TEMPERATURE OFTHE SIMICONDUCTOR TO A TEMPERATURE BELOW ITS FUSION POINT BUT ABOVETEMPERATURE AT WHICH THE SEMICONDUCTOR REMAINS PLASTIC TO CAUSE THESEMICONDUCTOR TO FREEZE NON-DIRECTIONALLY INTO A SOLID BUT PLASTIC MASS,RAISING THE TEMPERATURE OF A ZONE OF THE FROZEN SEMICONDUCTOR MASSADJACENT TO AND BOUNDED BY THE UPPER SURFACE THEREOF ABOVE THE FUSIONTEMPERATURE THEREOF WHILE MAINTAINING THE REMAINDER OF THE FROZEN MASSAT A TEMPERATURE BELOW ITS FUSION POINT BUT ABOVE THE TEMPERATURE ATWHICH THE SEMICONDUCTOR REMAINS PLASTIC TO ESTABLISH A MOLTEN ZONEADJACENT THE UPPER SURFACE THEREOF, CONTACTING THE SURFACE OF THE MOLTENZONE WITH A SEED CRYSTAL OF THE SEMICONDUCTOR, AND GROWING A SINGLECRYSTALLINE INGOT OF SEMICONDUCTOR BY SEED CRYSTAL WITHDRAWAL FROM THEMOLTEN ZONE WHILE REPLENISHING THE MOLTEN ZONE FROM THE UNMELTED PORTIONOF THE SEMICONDUCTOR MASS TO MAINTAIN THE MOLTEN ZONE AT A SUBSTANTIALLYCONSTANT VOLUME.